1. Field of Invention
This invention relates to a method of treating lignocellulosic or cellulosic pulp produced by chemical, semi-chemical, and chemimechanical types of pulping processes. More particularly it relates to the treatment of a lignocellulosic or cellulosic pulp with gaseous ammonia, which treatment promotes the kinking of pulp fibres and/or improves the tearing strength of paper prepared therefrom.
2. Description of the Prior Art
The kraft pulping process is a widely used chemical pulping process. Paper manufactured from kraft pulp is of good quality and is particularly characterised by high strength. However the kraft process is inherently highly polluting and the pulp is produced in a low yield, for example of about 45%. For purposes of this specification, the term "pulp yield" means the percentage of original dry wood material that is converted to dry pulp.
There are alternative high yield processes some of which are used commercially. Among these is the bisulphite process with which pulps have been produced at a yield in excess of 60%. There are in addition other high yield chemical or semi-chemical processes which have not yet achieved commercial acceptance. In addition less polluting processes, such as the two stage soda-oxygen pulping process have attracted considerable commercial interest.
As will be apparent from the above, alternatives to the kraft process can be attractive either because they are more acceptable environmentally or because they produce a greater yield of pulp. A disadvantage common to many of these alternative processes is that paper produced from the pulp resulting from these processes is a paper of low tearing strength. The other properties of papers produced from alternative pulps are in many cases either superior to or comparable with those of corresponding papers produced from kraft pulp. We have found that when a pulp prepared by a chemical or semi-chemical or chemimechanical process other than the kraft process is treated by the ammonia process according to the invention described and defined hereinbelow, there is an improvement in the tearing strength of the pulp so treated.
The gaseous ammonia treatment according to this invention also improves the tearing strengths of pulps produced by the kraft process and in this regard is particularly applicable to kraft pulps made from young, low density wood. Tearing strength is an important property in most end uses, particularly the manufacture of paper bags and sacks.
The treatment according to this invention has also been observed to induce and to set kinks in the pulp fibres. It is to be understood that what is meant by kinking of pulp fibres includes changes in the fibre configuration, such as, for example, in the extent of fibre twist, curl and kink as well as fibre wall dislocations, fractures, microcompressions and zones of dislocation. The presence of kinked fibres within a papermaking pulp is known to bring about an improvement in the properties of wet webs and in some of the papers produced from such webs. Kinked fibres are known to be particularly effective in developing extensibility in wet webs if the kinks are set in position so that they remain somewhat inflexible when the webs are subjected to strain during papermaking and dry lap production. Kinked fibres are also known to improve the extensibility of some papers produced from them.
Gaseous ammonia and aqueous ammonia solutions have been used as the alkaline reagent in oxidative delignification of lignocellulosic material and is described, for example, in British Pat. No. 1,381,728 and U.S. Pat. Nos. 3,617,432; 3,740,311; and, 4,002,526. Ammonia has also been used in conjunction with other gaseous reagents such as chlorine or chlorine dioxide to effect bleaching of wood pulp as is described, for example, in New Zealand Pat. No. 160,216, and U.S. Pat. No. 3,472,731.
In none of this prior art is there disclosed the use of ammonia in a separate treatment step in order to achieve the desired changes in the properties of the wood pulp being treated. The effects which gaseous ammonia has on wood pulp or other cellulosic fibres is unpredictable from any of the literature of which we are aware.
It is an object of this invention to go some way towards achieving the desiderata described above or at least providing the public with a useful choice.